Add extra installation steps for Ubuntu Focal

Add extra installations steps for installing Podman in Ubuntu Focal,
since it's not present in the official Ubuntu repos. This is the final
requirement to reinstate Ubuntu Focal support.

Closes #206

INSTALL.md

@@ -1,6 +1,7 @@
 Dangerzone is available for:
 
 - Ubuntu 22.04 (jammy)
+- Ubuntu 20.04 (focal)
 - Debian 12 (bookworm)
 - Debian 11 (bullseye)
 - Fedora 37
@@ -9,6 +10,27 @@ Dangerzone is available for:
 
 ### Ubuntu, Debian
 
+<details>
+  <summary><i>:memo: Expand this section if you are on Ubuntu 20.04 (Focal).</i></summary>
+  </br>
+
+  Dangerzone requires [Podman](https://podman.io/), which is not available
+  through the official Ubuntu Focal repos. To proceed with the Dangerzone
+  installation, you need to add an extra OpenSUSE repo that provides Podman to
+  Ubuntu Focal users. You can follow the instructions below, which have been
+  copied from the [official Podman blog](https://podman.io/new/2021/06/16/new.html):
+
+  ```bash
+  sudo apt-get install curl wget gnupg2 -y
+  source /etc/os-release
+  sudo sh -c "echo 'deb http://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_${VERSION_ID}/ /' \
+    > /etc/apt/sources.list.d/devel:kubic:libcontainers:stable.list"
+  wget -nv https://download.opensuse.org/repositories/devel:kubic:libcontainers:stable/xUbuntu_${VERSION_ID}/Release.key -O- \
+    | sudo apt-key add -
+  sudo apt update
+  ```
+</details>
+
 Add our repository following [these instructions](https://packagecloud.io/firstlookmedia/code/install#manual-deb), or by running this script:
 
 ```